Forgiveness Ratings

The Forgiveness Rating measures how much freedom the game gives you
to screw up the story. For example, in some games it's possible for
the main character to die by doing the wrong thing, while in others
there's no way to kill the main character. But dying is only the most
obvious way to "lose" in IF. In some games, you can get yourself into
a situation where you can go on playing forever, but you'll never be
able to win because of some irreversible action you've taken. For
instance, the game might let you lock your keys in the car, or it might
let you discard or destroy a vital object that you'll need to solve to
a puzzle much later in the game.

The Zarfian forgiveness scale was proposed by Andrew Plotkin on rec.arts.int-fiction to classify
games according to whether they make unwinnable situations possible,
and to what degree:

Merciful - You can't get stuck, and the main character
can't die. It's impossible to permanently lose an object that's
required to win the game. The game will always remain winnable no
matter what you do.

Polite - The main character can die, and it's possible to
lose vital objects or otherwise make it impossible to win, but
it's immediately obvious whenever you've just done something that
makes the game unwinnable. This means that you can always immediately
UNDO to get back on track.

Tough - You can get stuck or die, but it's immediately
obvious when you're about to do something irrevocable. (You might not
be able to anticipate that a particular action will make the game
unwinnable, though - only that it will be irreversible.)

Nasty - You can get stuck or die, but it's immediately
obvious after the fact when you've done something irrevocable.
(Again, though, you might not realize that an action made the game
unwinnable - just that the action was irrevocable.)

Cruel - You can get stuck or die, and you can do so by an
action that you didn't even realize was irrevocable after doing it.